Windows: Be a bit smarter when setting composite interfaces

Instead of open-coding the search for the "MI_" substring, use the
strstr() function to find the string and verify that the following two
characters are indeed digits.

Also guard against the possibility of the interface number being larger
than what we can support.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>

1 files changed, 11 insertions, 17 deletions

diff --git a/libusb/os/windows_winusb.c b/libusb/os/windows_winusb.c
index 7edb8e6..897d931 100644
--- a/libusb/os/windows_winusb.c
+++ b/libusb/os/windows_winusb.c
@@ -1186,31 +1186,25 @@ static void get_api_type(struct libusb_context *ctx, HDEVINFO *dev_info,
 static int set_composite_interface(struct libusb_context *ctx, struct libusb_device *dev,
 	char *dev_interface_path, char *device_id, int api, int sub_api)
 {
-	unsigned i;
 	struct windows_device_priv *priv = _device_priv(dev);
 	int interface_number;
-
-	if (priv->apib->id != USB_API_COMPOSITE) {
-		usbi_err(ctx, "program assertion failed: '%s' is not composite", device_id);
-		return LIBUSB_ERROR_NO_DEVICE;
-	}
+	const char *mi_str;
 
 	// Because MI_## are not necessarily in sequential order (some composite
 	// devices will have only MI_00 & MI_03 for instance), we retrieve the actual
 	// interface number from the path's MI value
-	interface_number = 0;
-	for (i = 0; device_id[i] != 0; ) {
-		if ((device_id[i++] == 'M') && (device_id[i++] == 'I')
-				&& (device_id[i++] == '_')) {
-			interface_number = (device_id[i++] - '0') * 10;
-			interface_number += device_id[i] - '0';
-			break;
-		}
+	mi_str = strstr(device_id, "MI_");
+	if ((mi_str != NULL) && isdigit(mi_str[3]) && isdigit(mi_str[4])) {
+		interface_number = ((mi_str[3] - '0') * 10) + (mi_str[4] - '0');
+	} else {
+		usbi_warn(ctx, "failure to read interface number for %s, using default value", device_id);
+		interface_number = 0;
 	}
 
-	if (device_id[i] == 0)
-		usbi_warn(ctx, "failure to read interface number for %s. Using default value %d",
-			device_id, interface_number);
+	if (interface_number >= USB_MAXINTERFACES) {
+		usbi_warn(ctx, "interface %d too large - ignoring interface path %s", interface_number, dev_interface_path);
+		return LIBUSB_ERROR_ACCESS;
+	}
 
 	if (priv->usb_interface[interface_number].path != NULL) {
 		if (api == USB_API_HID) {